One known gas sensor attached in use to, for example, an exhaust pipe of an automobile includes a sensor element which generates an electromotive force that changes with the concentration of a specific gas (e.g., NOx (nitrogen oxides) or oxygen) in exhaust gas or whose resistance value changes with the concentration. A sensing section for detecting the specific gas component is provided at the forward end of the sensor element. The sensing section is heated by, for example, a heater to detect the specific gas component. In the case where the sensing section of the sensor element is at high temperature, when water droplets contained in the exhaust gas adhere to the sensing section (the sensing section is wetted with water), the sensor element may break (for example, may crack) due to thermal shock. Therefore, a gas sensor in which the sensing section of the sensor element is covered with a protector to protect the sensor element from wetting with water has been developed (see, for example, Japanese Unexamined Publication No. 2009-115781).
As shown in FIG. 5, the gas sensor disclosed in Japanese Unexamined Publication No. 2009-115781 has a structure in which a sensor element 21 is inserted into and held within an insertion hole 320 of a ceramic holder 300 formed of an insulating material (ceramic such as alumina) and the ceramic holder 300 is disposed inside a metallic shell 1100. A sensing section at the forward end of the sensor element 21 is covered with a protective layer 25.
The metallic shell 1100 has a bore 1100h extending therethrough in a forward-rear direction, and a rear end portion 26 of the protective layer 25 of the sensor element 21 protruding forward from the ceramic holder 300 is accommodated within the bore 1100h. Metallic protectors 510 and 610 are attached to the forward end of the metallic shell 1100 so as to protect the sensor element 21. Gas passage holes 560 and 670 of the protectors 510 and 610 are arranged in the circumferential direction so as to be point-symmetric with respect to the center of the protectors.